Of the many ways devised to produce a printed page, none is more clever than the thermal printer. Known for inherent simplicity and quiet operation, this popular device burns tiny dots on heat-sensitive paper in graphic and character-shaped patterns.
Betting terminals seen at the racetrack are one common application. These terminals automatically process bet tickets somewhat the size of a business card by printing wagering information on the ticket as it slides across what is one of the most critical components of a thermal printer, the thermal printhead.
A typical printhead is a window pane thick ceramic plate, or "substrate", roughly 3.times.2 inches. Near one end lies a slender print element that looks like a straight line drawn across the substrate with an ink pen. This print element is electrically-resistive material deposited on the substrate, and it heats up much like a toaster element when electric current is forced through it.
Unlike a toaster element, the whole print element does not heat all at once. Instead, electrical circuitry forces current through very small print element segments to generate precisely controlled hot spots along the length of the print element. When a bet ticket driven across the printhead slides over the print element, as many as fifty hot spots per inch individually flicker on and off at just the right times to burn the desired pattern of dots into a heat-sensitive layer on the ticket. Thus, an inherently simple and quiet printing operation is performed.
But there are certain problems. One problem common to betting terminals and similar applications is printhead wear caused by bet tickets becoming soiled with abrasive material such as sand. As the ticket slides across the printhead, it acts much like a piece of sandpaper, and eventually printhead components are sanded down to the point where they require repair or replacement. This invention combats the problem.
Existing printheads are especially vulnerable to abrasive wear in several respects. First, a series of hair-thin wires, or "conductive traces", formed side-by-side on the substrate carry electric current down the length of the substrate from control circuitry to the print element. These traces are usually located on the surface of the substrate where the ticket slides. In this location they are easily damaged.
In addition, the print element is usually formed on the substrate near one end in a position lying across the conductive traces. It is above the level of the rest of the printhead and even more easily damaged.
So, abrasive damage to the conductive traces and print element is common, and it is not long before repair is required. One method of doing this is to cut off the worn print element end of the substrate, the last inch or so, and bond on a new print element end. Then each of the individual traces is reconnected and the printhead is put back in service, but only after expending significant time and expense.
An alternate repair approach involves replacing the entire substrate. But since associated electronic circuitry is often built upon the substrate, this too is a costly method.
Consequently, it is desirable to have a new and improved printhead that remains in service longer before repair or replacement is necessary.
It is desirable that the printhead include wear-resistant features that resist abrasive wear caused by impacting tickets.
It is desirable that the wear-resistant features be adaptable for use on existing printheads. And, it is desirable that the wear-resistant features be compatible with existing thermal printer designs.